Methods and Compositions for Preparing Particle Boards

ABSTRACT

An amorphous polylactic acid polymer having a weight average molecular weight in the range of about 35,000 to 180,000 is described. The polylactic acid polymer composition can be hammer milled without cryogenics result in the form of particles wherein 90% of the particles have particle size of about 250 μm or less and the material has a glass transition temperature of between about 55° C. to about 58° C. and a relative viscosity of about 1.45 to about 1.95 centipoise. The polymer composition can be used to form an aqueous suspension. The material is ideally suited for use in preparing particleboard. A method is disclosed for preparing such polylactic acid polymers. The method involves obtaining an amorphous polylactic acid polymer having a weight average molecular weight of between about 115,000 to about 180,000. Treating the polylactic acid polymer to reduce the molecular weight to between about 35,000 to 45,000 such that it has a glass transition temperature of between about 55° C. and 58° C. and a relative viscosity of about 1.45 to about 1.95. Material can be formed into particles in a commercial hammer mill with bypass such that 90% of the initial mass results in the particles which can pass thru a sieve having a pore size of about 250 μm. During particle board formation the temperature of around 140-140 C being reached to optimally activate the adhesive; Bond strengths and throughput rates of resulting particle boards can be controlled thereafter, with variable combination of particle sizes, adhesive loading and initial moisture content.

BACKGROUND

Natural adhesives such as animal glues, fish glues, vegetable glues andcasein (the main protein in milk) are generally set upon solventpreparation and offer low strength and are susceptible to moisture andmold. Their use is mainly for joining low strength materials.

Elastomer adhesives such as natural rubbers, neoprene, acrylonitridebudadiene, butyl/rubber adhesives, styrene butadiene rubber adhesives,polyurethane adhesives, polysulfide rubber adhesives, and siliconerubber adhesives are based on natural and synthetic rubbers set bysolvent evaporation or heat curing; they have relatively low strengthand suffer from creep and therefore are not usually used for stressedjoints. More typically, they are used for flexible bonding of plasticsand rubbers.

Thermoplastic adhesives such as polyvinyl acetate (PVA), polyvinylalcohol (PVA), polyacrylates, polyester acrylics, acrylic solventcement, cyanoacrylates (superglue), silicone resins, polyamides andacrylic acid diesters have low/medium strength and may suffer from creepand attack from water but not from oils.

Thermoset adhesives such as urea formaldehyde (UF), phenol formaldehyde(PF) resins, phenolic neoprene, polyesters, polyamides and epoxy resinsset as a result of the buildup of rigid molecular chains withcross-linking under various conditions of temperature and moisture.

Toughened rubber modified adhesives include small rubber-like particlesdispersed throughout a glassy matrix are resistant to crack propagationand have been applied to acrylic and epoxy-based adhesives.

The general performance characteristics of these adhesives in terms ofshear strength and range of operating temperatures are summarized in thetable below.

TABLE 1 Properties of conventional adhesives Shear Operating strength(MPa) temperature (C.) Adhesive Type Min. Max. Min. Max. Rubber 0.35 3.5−20 150 PVA (white glue) 1.4 6.9 Cyanoacrylate 6.9 13.8 80 Anaerpbo 6.913.8 200 Polyurethane 6.9 17.2 −200 150 Rubber modified 13.8 24.1 −40 90epoxy Epoxy 10.3 27.8 200 Polyamide 13.8 27.6 350 Rubber modified 20.741.4 180 epoxy Note: 1 MPa = 10 bar about 147 psi; 1,000 psi = about 6.9MPa

The strength of adhesives is dependent on how well the adhesive hasbonded to the surface of a material (i.e., substrate), as well as on thecohesive strength of the adhesive itself. Virtually all of the listedcompositions emit volatile organic compounds (VOCs) and they require settimes ranging from tens of minutes to days, and the operatingtemperatures are generally below 150° C. to 200° C.

One area where novel adhesives are needed is in the area of particleboard manufacture. Examples of boards in use today include laminateflooring. Laminate flooring can be prepared by coating an adhesive ontowood particles or floors at high temperature, followed by molding andhot-pressing. Since the laminate flooring can be subjected tocomplicated machining and the like, the laminate flooring is widely usedfor interior finishing or overall furniture products. New adhesives areneeded that can be used in the manufacture of particle boards.

Currently the adhesive used to make particle boards is mainlyurea-formaldehyde resin or a melamine-urea-formaldehyde resin. Theseadhesives exhibit good adhesion and are low-priced but the adhesive canirritate the eyes, nose and skin, as well as causing atopic diseases andbronchial asthma even after curing, and gradually emits formaldehyde,which can cause cancer when inhaled for a long time. In addition, excessmelamine intake can result in formation of kidney stones in humans.Further, melamine, urea, formaldehyde and the like, which are preparedfrom fossil resources can be subject to price appreciation as fossilresources become depleted. Moreover, their production is associated withthe emission large amounts of greenhouse gases and they consume a largeamount of energy to prepare. During particle board manufacturing the useof urea formaldehyde adhesives requires use of scavengers and VOC reliefequipment which can lead to industrial safety, handling and cost issues.Lastly, they are known to emit a variety of toxic substances such asendocrine disruptors, toxic gases and the like, when incinerated.

New adhesives are needed that have improved fatigue behavior and reducedstress concentration zones. They should be easy to use, allowing forhigh through put and have sealing capability such that the adhesivejoint can seal joined materials from moisture and air. In addition, theyshould not melt or otherwise modify the substrates they are intended tojoin, they should be amenable to use in joining a broad range ofsubstrates including aluminum substrates and they should be of minimaldensity.

SUMMARY OF INVENTION

An amorphous polylactic acid polymer having a weight average molecularweight (Mw) in the range of about 35,000 to 180,000 is described. Thepolylactic acid polymer in treated form using either of thermal,hydrolysis or ionizing beam methods a resulting composition of MW in the35,000 to 45,000 range which can be reduced to particles by single passcommercial grade (e.g., Fitzpatrick) hammer milling with dynamicscreening and without cryogenics wherein 90% of the particles haveparticle size of about 250 μm or less and the material has a glasstransition temperature of between about 55° C. to about 58° C. and arelative viscosity at 30° C. is about 1.45 to about 1.95 and morepreferably 1.45 to about 1.6. The polymer composition can be used toform an aqueous suspension or used directly in dry form. The material isideally suited for use in preparing particleboard.

A method is disclosed for preparing such polylactic acid polymers. Themethod involves obtaining an amorphous polylactic acid polymer having aweight average molecular weight of between about 115,000 to about180,000 and more preferably about 140,000 and possessing relativeviscosity (based on dilution viscometry) of about 2.5. Treating thepolylactic acid polymer to reduce the molecular weight to between about35,000 to 45,000 and more preferably about 43,000 such that it has aglass transition temperature of between about 55° C. and 58° C. and arelative viscosity based on dilution viscometry of about 1.45 to about1.60. Material can be formed into particles such that 90% of theparticles have an average diameter of less than 250 μm. This can becarried out using industrial type hammer milling equipment and can beaccomplished in a single pass with dynamic screening. Importantly, thiscan be accomplished without resorting to cryogenic cooling.

The molecular weight and relative viscosity of the starting amorphoustype polylactic acid polymer can be reduced from about 140,000 to180,000 with a glass transition temperature of about 67° C. and arelative viscosity of about 2.5 down to a molecular weight of about44,000 and a relative viscosity of about 1.5 by hydrolysis in a constanttemperature water bath at about 92° C. for about 8.5 hours.Alternatively, the starting high molecular weight polymer can beprocessed thermally at a temperature of 255° C. for a sufficient time ofabout 45 minutes to obtain the desired molecular weight and viscosity inglass transition temperature. The molecular weight can also be reducedby treating material with radiation such as with Co⁶⁰ for a dose ofabout 100 kGy or with an eBeam a dose of about 200 kGy or alternately,in equivalent fashion using UV radiation.

Suitable polymers can also be produced directly via ring polymerizationby continually building up the size of the polymer chain starting with Dand L-form lactides starting from corn-derived dextrose. A similar butcontrolled pathway can be taken to adjust the process parameters toarrest further polymerization once the desired combination of molecularweight of about 43,000 and relative viscosity of about 1.5) and having aglass transition temperatures in the 55-58° C. range. This can beaccomplished from dynamic monitoring of molecular weight and relativeviscosity of the polymer during the polymerization

DETAILED DESCRIPTION OF INVENTION

The term “about” means within 10%.

The term “molecular weight” refers to a weight average molecular weightfor purposes of this specification.

A polylactic acid polymer composition is disclosed. The polylactic acidpolymer is amorphous and has a weight average molecular weight in therange of about 35,000 to 45,000 for enabling sufficient strength butalso enablement for single-pass (about 90% of the feed mass) powderproduction in a commercial grade hammer mill—example FitzpatrickHammermill. Alternately, for ˜50% higher end strength of the particleboard the weight average molecular weight may be higher towards about120,000 with a glass transition temperature of about 65-70 C, but forwhich hammer mill based pulverizing without cryogenics may requiremultiple 2-3 passes—a feature that may be overcome by conducting themilling under cryogenic (liquid nitrogen temperature type) conditions.In an embodiment the treated material has a particle size sufficient topass through a sieve having a pore size of about 250 μm or less whichcan be formed without resort to cryogenics during hammer milling in asingle pass. In an embodiment the treated polymer composition has aglass transition temperature of between about 55° C. to about 58° C. Inan embodiment the treated material has a relative viscosity of thematerial at 30° C. of about 1.45 to about 1.95 and more preferably about1.45 to about 1.6.

The polylactic acid polymer can contain about 10 to about 15 molepercent D-lactide and can be prepared from an amorphous polylactic acidpolymer having a higher molecular weight or can be synthesized bypolymerizing lactic acid using well known means. For example, acommercial polylactic acid known as 10361D® can be purchasedcommercially from Natureworks. That polymer can then be made into apolymer powder form with 90% of the initial mass run down below about250 microns in a single pass using a commercial grade (e.g.,Fitzpatrick) hammer mill with continual bypass and without resort tocryogenic type cooling-having suitable characteristics for particleboard manufacture by any suitable means. Several alternate crushingmeans are also contemplated, e.g., by placing the multi-mm polymer resinin a blender, or subjecting to impact loads or expulsion from highpressure nozzles as used for atomization. Amorphous polylactic acidpolymers having a molecular weight in the range of about 115,000 toabout 180,000 can also be used to provide roughly 30-50% higher internalbond strengths of the plywood but they are not as easily pulverized. Theprocess of milling or pulverizing leads generally in nature to sizedistribution (often in the log normal variety) which can becharacterized by a 90% upper bound size (e.g., 90% of the initial massground down such that the highest size is about 250 microns). The impactof varying the polymer powder (upper bound) size on resulting particleboard strength is also specified elsewhere in this application.

In one method the 10361D® polymer starting material can be suspended inwater and heated at a constant temperature of about 92° C. for about 8.5hours. Lower temperatures can be used for longer periods of time asdesired. Temperatures significantly higher than about 94° C. result inpellet clumping. The material can then be dried by any suitableconventional means such as by heating in an oven at about 50° C. for20-24 hours to reduce the moisture content and then substantially (90%+feed) ground in a single pass using to the desired particle size ofunder 250 microns, such as by a commercial grade Fitzpatrick hammermill. The preparation of a material that can be sieved thru a 250 micronor smaller sieve provides for the preparation of stable aqueoussuspensions that can be used interchangeably with current melamine orformaldehyde based adhesives used in current particle boardmanufacturing operations without clogging aqueous adhesive emulsionspray nozzles or mixed directly in dry form with furnish. The resultingparticle board has water resistance characteristics and a suitablestrength for widespread use in the particle board manufacturingindustry.

Natureworks 10361D® polylactic acid can also be processed by heating andmelting the ˜2-3 mm resin beads at a constant temperature of about 255°C. for about 45 minutes taking care to minimize oxidation at themelt-air interface (or alternately, to do the melting in an inertedenvironment). This heating process causes the polymer to melt and clumpbut causes a reduction in molecular weight and relative viscosity.Therefore, once the heating is discontinued processing the chunkymaterial to pass through a mesh sieve (of the desired range—e.g., 250micron) is more difficult. This can be alleviated by making pelletsduring melting by extrusion processes or by cooling in a dimpled pan orcrushing. Once the material is processed to a suitable form and size itcan be introduced into a commercial grade hammer mill and ground into asuitably sized powder in a single stage. Generally, a powder of havingan average crosssection of no more than about 250 microns is desired;smaller particle sizes such 200, 150, or even 100 microns or less arealso contemplated in order to more evenly spread and stick to the woodparticulates in the furnish resulting in higher strength.

Natureworks 10361D® polylactic acid can also be processed using anirradiation process such as by treating the polymer with about 100 kGyof Co⁰ gamma irradiation or about 200 kGy with an electron beam and thenprocessed to form particles by (Fitzpatrick grade) hammer milling in asingle stage without cryogenics that can then substantially (˜90%) passthrough a 250 micron sieve or smaller.

The ground polylactic acid can then be used to prepare an aqueoussuspension. Any concentration of polylactic acid can be used that canprovide enough adhesive in the suspension for convenient use in particleboard manufacture but which does not allow for particle agglomerationduring storage. For example, about 10 wt. % to about 75 wt. % can beused, more preferably about 20 wt. % to about 55 wt. %, even morepreferably about 25 wt. % to about 40 wt. % and generally a weightpercent of about 33 to about 35 is envisioned. For suspensions havingmore than 55 wt. % solid content particle sizes below about 125 micronsare used to avoid adhesive particle coagulation can be achieved by usinga suitable surfactant.

A surfactant such as Triton X-100 can be added to the water to helpprevent particle agglomeration. For example, from about 0.3 wt. % to 10wt. % of a surfactant can be added to the water followed by addition ofthe prepared polylactic acid polymer particles. The mixture can be mixedto form the suspension. A suspension made in this manner will remain asuspension for from several days to weeks without significant settlingor coagulation. In the event that settling does occur the suspension canbe easily reformed by mixing.

The suspension made as described herein can be used in place ofconventional formaldehyde and melamine adhesive preparations for theproduction of particle board without the need for significant changes tothe equipment used for particle board manufacturing.

Particleboard or chipboard can be manufactured by mixing wood particlesor flakes together with a resin and forming the mixture into a sheet.The raw material to be used for the particles can be prepared by anysuitable means, such as by feeding it into a disc chipper with radiallyarranged blades. The particles are then dried, after which any oversizedor undersized particles can be screened out resulting in what isreferred to as furnish.

An adhesive resin such as the suspension of or in dry powder form of thepolylactic acid polymer described herein is then mixed into or sprayedthrough nozzles onto the particles. Mixing is performed using variety ofconventional mixing methods including: shaking in a bag, tumbling inrotary mixer, or placement in a vessel and using rotary paddles. Due tocolor variations of resin powder to that of furnish any localizedclumping of resin powder or segregation can be noticed by the naked eyeas well as via examining random samples of mixtures for uniformity viarelative ratios of furnish to powder masses.

Various other chemicals can also be added to the particle boards duringmanufacture including wax, dyes, wetting agents, release agents. Theseadditives can be used to make the final product water resistant,fireproof, insect proof, or to give it another desirable quality.

Once the resin has been mixed with the particles, the mixture is madeinto a porous sheet. A weighing device notes the weight of flakes, andthey are distributed into position by rotating rakes. In graded-densityparticleboard, the flakes are spread by an air jet that throws finerparticles further than coarse ones. Two such jets, reversed, allow theparticles to build up from fine to coarse and back to fine.

The sheets formed are then compressed to mats to reduce their thicknessand make them easier to transport. Later, they are compressed again,under heat (at platen temperatures ranging from about 150° C. towards220° C.) pressures between 2 and 4.2 megapascals (290 and 600 psi).Platen temperatures above 220 C are not recommended due to significantburning of the wood (the avoidance of which may be accomplished by usingan inerted environment, e.g., use of argon or nitrogen enrichedambient). A non-stick Teflon type sheet is preferably placed between themetal plate and the mat during board formation to minimize stickingrelated wear and unevenness upon pulling up the platen. All aspects ofthis entire process must be carefully controlled to ensure the correctsize, density and consistency of the board. Higher temperatures andpressures reduce the time required for particle board formation and aredesirable for enhanced throughput if explosive blowouts can beprevented. Conventional thermoset adhesives such as urea formaldehydeset and harden while being processed and can give rise to blowouts andhence require lower temperatures and pressures during board formation.The thermoplastic nature of the reduced molecular weight and relativeviscosity amorphous polymer of this invention permits faster throughputblowout free particle board formation. The optimally reduced viscosityat temperatures from 140° C. to as high as 220° C. permit steam reliefwithout blowout once the overpressure is removed. When using theadhesive (of MW in range of about 35,000-140,000) as described in thisinvention, attaining the temperature of about 140-145 C or higherthroughout the matrix is preferred in order to activate that adhesiveprior to pressure relief and cooldown to form particle boards. While theouter edges of the particle board next to the hot platen take less timeto reach to about 140-145 C, the central core region can take longer forwhich a pre-calibration must be done for the furnish type, desired enddensity, amount of resin and also, the initial moisture content.Moisture content can be expected to play a significant role becauseboiling off water occurs at its saturation temperature, is endothermic,only after which the matrix temperature can substantially rise towardsthe desired 140-145 C range or above. For example, the time requiredunder the platen varies with thickness and density of the board. Fornominal density in the ˜0.75 g/cc range and thickness of ¼″ undertypical ˜10% moisture content; ˜10 w/o of adhesive and platentemperatures and pressure settings in an industrial grade Wabash mfg.style press of about 220 C and 600 psi respectively, the time it wouldtake the centerline temperature to reach ˜140-145 C can be expected tobe about 2 minutes or less; thicker boards and higher moisture contentand density can require longer times. The resulting particle boardsprovide adequate performance similar to or superior to those ofcommercial grade particle boards made with urea formaldehyde and otherconventional VOC bearing resins: viz., per ASTM D1037; ANSI A208.1-2009using an Instron 556A load frame tester—modulus of rupture (Rb) s ofabout 7 MPa (1 ksi), modulus of elasticity in the 700 MPa (100 ksi)range and internal bond (IB) strength above 0.14 MPa (20 psi) towardsand above 0.7 MPa (100 psi). The IB value can be made to vary dependingon adhesive loading and particulate size. For example, for a nominalloading of about 8.5 w/o of adhesive, ˜12% moisture content and use ofmilled adhesive particles of size below 250 microns one may attain IB˜0.7 MPa (100 psi) range. If coarser particles are used the degree ofadhesive spread with the furnish to cause adhesion can result in lowerstrength and IB values. For example, for ¼″ to ⅝″ type range thickboards starting with about 10 w/o to 15 w/o resin for a resultant ˜0.75g/cc board: if milled particles below 500 microns (i.e., 2×250) areused, the IB can be expected to reduce by a factor of ˜2 down to ˜0.35MPa (˜50 psi); with 1000 micron diameter and below, a further reductionof ˜2 may be expected to ˜0.175 MPa (˜25 psi) and so on but goingsignificantly above, e.g., to <2,000 micron powders now can suddenlyresult in virtually sharp reductions in IB towards ˜<0.01 MPa (<1 psiatype values). Such reduction in IB may be compensated by increasing theinitial adhesive resin loading—which can produce nonlinear increases inIB values. For example, for a nominal case (e.g., 12% moisture; 0.75g/cc final density) if the adhesive content is increased towards 20-30%,the IB value may more than double towards 200-300 psia range. Producingboards under this method can result in thicker boards with lowerinternal bond strengths than thinner boards, an aspect which can becontrolled by changing the aspect ratio during hot compression.

Methods for repairing and reusing particle boards made from thedisclosed polylactic acid polymers are also contemplated. For example, acracked particle board can be repaired by directly subjecting thedamaged particle board to compression and heating for example at 4.2 MPa(600 psi) and 220° C. platen temperature as discussed earlier, for timecommensurate with bringing the internal board to temperatures at orabove about 140-145° C. to reactivate full adhesion strength uponcooldown. The resulting board then possesses similar strength andfunctionality of the original board.

Another means for particle board formation with the polymer adhesivedescribed herein is to prepare the mats as described earlier and insteadof hot compressing the mat under platens as is the norm, the mat mayinstead be placed in a heated enclosure like in an oven or via radiantlamps or induction heating, to bring the mat contents up towards about140-145 C or higher to activate the adhesive for highest strength,followed with stamping compression under loads and traverse commensuratewith reaching the desired density and thickness. Significant woodburning or charring should be minimized by control of the ambient oxygencontent (e.g., inerting by use of argon or nitrogen) or placing asuitable barrier/film. The stamp machine temperature would preferablyremain at or above the glass transition temperature for the polylacticbased thermoplastic adhesive described herein. During stamping anon-stick layer such as a Teflon or ptfe film may be used to avoidlocalized sticking and ensuring a smooth surface. Below about 100 C theamorphous adhesive tends to harden sufficiently such that once the boardtemperature is below this level the external compression may be relievedand the board allowed to cool down to ambient temperature.

1. A polylactic acid polymer composition comprising an amorphouspolylactic acid polymer having a weight average molecular weight in therange of about 35,000 to 180,000, wherein the material has a particlesize sufficient to pass through a sieve having a pore size of about 250μm or less, wherein the material has a glass transition temperature ofbetween about 55° C. to about 58° C., and the relative viscosity of thematerial is about 1.45 to about 1.95.
 2. The polylactic acid polymer ofclaim 1 wherein the relative viscosity is from about 1.45 to about 1.6.3. An aqueous suspension that remains stable for over 30 days withoutcoagulation comprising the polylactic acid polymer composition of claim1 together with a surfactant in which the surfactant is about 0.3 wt. %to 1 wt. %, the suspension containing about 55 wt. % of polylactic acidpolymer.
 4. An aqueous suspension that remains stable for over 30 dayswithout coagulation comprising the polylactic acid polymer compositionof claim 1 together with a surfactant in which the surfactant is about0.3 wt. % to 1 wt. %, the suspension containing about 70 wt. % ofpolylactic acid polymer having a particle size sufficient to passthrough a sieve having a pore size of about 125 micron powders.
 5. Aparticle board comprising the polylactic acid polymer composition ofclaim
 1. 6. A method for preparing a polylactic acid polymer compositioncomprising obtaining an amorphous polylactic acid polymer having aweight average molecular weight of between about 115,000 to about180,000 and forming into powders having an average diameter below 250μm; alternately, treating the polylactic acid polymer such that themolecular weight is between about 35,000 to 45,000, it has a glasstransition temperature of between about 55° C. and 58° C., and it has arelative viscosity of about 1.45 to about 1.95 centipoise; for formingparticles from the treated polylactic acid polymer with grinding withoutcryogenics wherein 90% of the particles have a diameter of less thanabout 250 μm; thereafter, mixing with furnish to form mats.
 7. Themethod for preparing a polylactic acid polymer composition of claim 6wherein the relative viscosity is from about 1.45 to about 1.6.
 8. Themethod for preparing a polylactic acid polymer of claim 6, wherein thetreating step includes hydrolysis in a constant temperature water bathat about 92° C. for about 8.5 hours.
 9. The method for preparing apolylactic acid polymer of claim 6, wherein the treating step includesthermal processing and the constant temperature bath at about 255° C.for about 45 minutes.
 10. The method for preparing a polylactic acidpolymer claim 6, wherein the treating step includes irradiation with aradiation source.
 11. The method for preparing a polylactic acid polymerof claim 6, wherein the treating step includes irradiation with Co⁶′ fora dose of about 100 kGy
 12. The method for preparing a polylactic acidpolymer claim 6, wherein the treating step includes irradiation with aneBeam for a dose of about 200 kGy
 13. A method for preparing a particleboard comprising mixing a polymer composition of claim 6 with a woodfurnish, forming a mat and subjecting the mat to a pressure of betweenabout 400 to 600 psi and a temperature of from about 180° C. to about220° C. in a platen press for a time span sufficient for the matrix ofthe mat to reach at or around 145° C. for prior to raising of theplatens.
 14. A method for preparing a particle board comprising mixing apolymer composition of claim 6 with a wood furnish, forming a mat andsubjecting the mat to a thermal environment to raise its bulktemperature to around 145° C. and then subjecting the heated mat tocompression load for desired density and thickness followed withpressure relief upon board having cooled below about 100° C. or glasstransition temperature.
 15. A method for preparing particle boardcomprising steps as in claim 13 using varying sizes of powders rangingup to 2,000 microns in average diameter to attain enable boardspossessing internal bond strengths that reduce proportionately withincreasing particle size.
 16. A method for repairing a particle board bydirectly pressing the damaged particle board with a pressure of betweenabout 400 to 600 psi and a temperature of from about 180° C. to about220° C. in a platen press for time span sufficient to raise the bulk ofthe board to around 140-145° C. prior to pressure relief and cooldown.